1. Field of the Invention
The present invention relates to a display device or a so-called multi-view display device capable of displaying individual images for a plurality of viewing directions on a common screen. The multi-view display device is capable of offering individual and different data substantially simultaneously for a plurality of users on a single screen. The invention further relates to a method of adjusting the image quality in a display device which is capable of displaying individual images for a plurality of viewing directions on the common screen. The invention further relates to a device for adjusting the image quality of a display device or a so-called multi-view display device which is capable of displaying individual images for a plurality of viewing directions on the common screen. The invention further relates to a device for adjusting the contrast having an A/D converter unit that converts analog image signals into digital image signals and to a display device and, more specifically, to a device for adjusting the contrast for individual images displayed on the common screen by varying a reference voltage at the time of A/D conversion.
2. Description of the Related Art
A display device or a so-called multi-view display device has been realized, which is capable of offering individual images to the observers on the right and left sides through the same display panel. The following patent document No. 1 is disclosing a device of this kind. This device enables individual images to be observed from the left side and the right side of the display panel without using spectacles for viewing three-dimensional images. To use the display panel, a method is also provided to offer different images to a plurality of users simultaneously.
Further, patent documents No. 2 and No. 3 are disclosing display devices capable of simultaneously displaying two images on one liquid crystal display (LCD), e.g., a display device which enables different images to be seen from the driver's seat and from the assistant driver's seat. Further, patent documents No. 4 and No. 5 are disclosing two-screen display devices for simultaneously displaying two kinds of images on a common screen.
Typically, the display unit is constituted by using a liquid crystal panel having a parallax barrier as disclosed in patent documents No. 6 and No. 7 enabling an observer (user) positioned on the left side of the display unit (e.g., on the side of the assistant driver's seat) to see only those pixels corresponding to a first display image but substantially preventing a second display image from being seen due to the parallax barrier. On the other hand, an observer (user or driver) positioned on the right side of the display unit (e.g., on the side of the driver's seat) is allowed to see only those pixels corresponding to the second display image but is substantially prevented from seeing the first display image being interrupted by the parallax barrier. There is thus realized a display device enabling different screens to be seen from the driver's seat and from the assistant driver's seat.
Further, patent document No. 8 is realizing the above multi-view display device by alternately arranging pixels of two different images on a horizontal scanning line in the liquid crystal screen, by alternately providing slit barriers having a slit maintaining a predetermined pixel width in the longitudinal direction of the screen in front of the liquid crystal screen, and by individually displaying two images on the liquid crystal screen so as to appear differently depending upon the viewing positions.
Further, patent document No. 9 discloses an image display device which selectively offers necessary images in a direction in which an operator is present, and selectively limits the display of particular images for a predetermined operator depending upon the conditions occurring on the outer side.
When the above conventional display device or the so-called multi-view display device is mounted in a vehicle, navigation information can be displayed on the display panel of the display device for a driver on the driver's seat while displaying TV images or DVD (digital video disk) images on the display panel for a person sitting on the assistant driver's seat. Therefore, the driver on the driver's seat is allowed to devote himself to the driving while the person on the assistant driver's seat does not have to feel boring; i.e., the display device can be favorably used being mounted on the vehicle.                (1) Patent document No. 1: JP-A-2004-206089        (2) Patent document No. 2: JP-A-6-186526        (3) Patent document No. 3: JP-A-2000-137443        (4) Patent document No. 4: JP-A-11-331876        (5) Patent document No. 5: JP-A-9-46622        (6) Patent document No. 6: JP-A-10-123461        (7) Patent document No. 7: JP-A-11-84131        (8) Patent document No. 8: Japanese Patent No. 3194024 (particularly, FIG. 3)        (9) Patent document No. 9: JP-A-2004-233816        
As described above, the multi-view display device is usually used in a manner of enabling different users to watch different images. Therefore, values for adjusting the images such as color tones may differ for each of the images. That is, each user wants to set the quality of the image which he watches depending upon his liking. However, the conventional image-adjusting circuit has been designed to adjust the image such as color tone of the display panel as a whole, and various display settings (brightness, contrast, sharpness, color tone, etc.) for adjusting the image are allowed to be effected for one display panel only. As a result, it was not allowed to set optimum or desired display quality for the images that were individually observed from the driver's seat side and from the assistant driver's seat side.
For example, glare on the screen may differ depending on the seats due to the direction of incidence of sunlight. In the conventional multi-view display device, the image could be adjusted for only the display panel as a whole; e.g., even if the image is just desirable for the driver's seat side, the image may be too bright for the assistant driver's seat side.
More concretely, when the conventional multi-view display device is applied to a car navigation device, it is desired to provide the multi-view display device with a so-called contrast-adjusting function for adjusting the brightness or darkness (whiteness, blackness) for both images displayed for the driver's seat and the assistant driver's seat since the vehicle travels under varying conditions.
It will become more convenient if the above contrast-adjusting function could be utilized for a variety of devices in addition to the multi-view display device.